Method for inhibiting the nitridation of magnesium



Patented May 1, 1934 PATENT OFFICE IVIETHOD FOR INHIBI'I'ING THE N ITRIDA- 'I'ION OF MAGNESIUM Edwin o. emcee, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Michigan No Drawing. Application December 14, 1932, Serial No. 647,245

2 Claims.

The invention regards a method of inhibiting the action of nitrogen upon heated magnesium.

The term magnesium as used herein and in the appended claims means elemental magnesium 5 or alloys thereof wherein the magnesium content predominates. The term heated as applied herein to magnesium means that the temperature of the magnesium is between about 500-1500 F., i. e. in the range normally employed for processing magnesium in operations such as electrolytic production, melting, alloying, pouring, casting in sand or other molds, pressure casting, heat treating, and other comparable operations known to those skilled in such art.

In processing heated magnesium it is well known that air must be excluded from contact therewith in order to prevent the destructive at-' tack of the oxygen therein upon the heated metal.

It has therefore been necessary to develop protective atmospheres wherein heated magnesium can be processed; and for this purpose it has been proposed to use combustion gases substantially free from uncombined oxygen and high in nitrogen content, hereinafter termed a nitrogen atmosphere, inasmuch as such gases can be produced very economically. Attempts to do so however were unsuccessful because it was found that while the effect of the oxygen was eliminated,

that nitrogen in contact with heated magnesium reacts therewith to form magnesium nitride, the reaction progressing with increased vigor as the temperature of the metal is raised. This fact has, then, heretofore served as an efiective bar to the utilization of a nitrogen atmosphere in the processing of heated magnesium. Since impure nitrogen, e. g. combustion gases, can be readily produced by burning coke or other fuel in air, a method whereby heated magnesium being processed might be protected by such an atmosphere is greatly to be desired.

It is therefore an object of this invention to provide a method for inhibiting the reaction between heated magnesium and nitrogen in contact therewith whereby the processing of heated magnesium in an atmosphere substantially free from uncombined oxygen but containing elemental nitrogenis made possible. The invention, then, consists of the method hereinafter fully described and particularly pointed out in the claims.

I have now found that reaction between heated magnesium and nitrogen can be substantially inhibited by incorporating sulphur with the nitrogen. A relatively small amount of sulphur suffices to suppress the nitrogen-magnesium reaction,'e. g. between about 1 and about 10 parts of sulphur vapor per 100 parts of nitrogen, by volume; although the use of other proportions of sulphur does not depart from the scope of my invention. The effectiveness of the sulphur in do inhibiting the nitrogen-magnesium reaction is not affected even when there is mixed with the nitrogen a small amount, e. g. up to about 20 per cent by volume, of gases such as carbon dioxide, carbon monoxide, sulphurdioxide, etc.; but the on nitrogen atmosphere is preferably to be free from water vapor. The drying of the atmosphere may be accomplished in any suitable manner, e. g.

contact with a hygroscopic salt, or solution there- The manner of incorporating the sulphur with the nitrogen prior to contacting it with the heated magnesium is susceptible of wide variation. For example, the sulphur may be vaporized and the vapors thereof commingled with the nitrogen prior to contact thereof with the heated magnesium. Another convenient method is to maintain a. body of sulphur in the liquid condition and pass the nitrogen through or over the same, sufficient sulphur to inhibit the reaction of the nitrogen upon magnesium being carried along by the nitrogen due to the vapor pressure of the sulphur. For example, at 240 0., about 1.1 parts by volume of sulphur vapor will be entrained by 100 parts of nitrogen passed therethrough. Other suitable as methods of application will be readily apparent to those versed in such art.

The manner of utilizing a protective atmosphere substantially free from uncombined oxygen and consisting essentially of nitrogen having incorporated therewith a relatively small amount of sulphur, for the protection of heated magnesium being processed may be described broadly by stating that the air normally in contact with the metal is displaced by the protective atmosphere and the air thus excluded from contact with the metal. As specific examples of such procedure I mention the melting of magnesium in a substantially covered crucible wherein the space over the heated metal is filled with the protective atmosphere supra; the heat treating of magnesium articles in furnaces wherein the air has been displaced by the protective atmosphere; and, the use of such an atmosphere to force the molten magnesium from the melting crucible into the mold in pressure die-casting apparatus.

Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the method herein disclosed, provided the step or 110 m bined oxygen and containing sulphur.

2. The method of protecting heated magnesium during processing which comprises maintaining in contact with the magnesium a nitrogen atmosphere substantially free from uncombined oxygen and containing a relatively small amount of sulphur.

EDWIN 0. BARSTOW- 

